NullReferenceException: Object reference not set to an instance of an object (wrapper stelemref) object:stelemref (object,intptr,object)

using UnityEngine; using System.Collections;

public class spawn : MonoBehaviour { public GameObject wisp; public GameObject player; public GameObject enemy1; public GameObject enemy2; private GameObject[] wisps; int counter=0; int playerScore=0; int enemy1Score=0; int enemy2Score=0; float countdown= 5;

 // Use this for initialization
 void Start () 
 {
     Instantiate( enemy1,new Vector3(Random.Range(40,400),10,Random.Range(40,400)),Quaternion.identity);
     Instantiate( enemy2,new Vector3(Random.Range(40,400),10,Random.Range(40,400)),Quaternion.identity);
 
 }
 
 // Update is called once per frame
 void Update () 
 {
     if(countdown < Time.time)
     {
         countdown = Time.time +5F;
         wisps[counter]= Instantiate( wisp, new Vector3(Random.Range(40,400),10,Random.Range(40,400)),Quaternion.identity)as GameObject;
         counter++;
     }

I realy dont understant what the problem is and I'm about to get mad.